PostgreSQL utilizes lightweight locks(LWLocks) to synchronize and control access to the buffer content. A process acquires an LWLock in a shared mode to read from the buffer and an exclusive mode to write to the buffer. Therefore, while holding an exclusive lock, a process prevents other processes from acquiring a shared or exclusive lock. Also, a shared lock can be acquired concurrently by other processes. The issue starts when many processes acquire an exclusive lock on buffer content. As a result, LwlockAcquire seen as top hot function in profilng.
Here need to understand LwlockAcquire is lock contention or cpu time spent inside the method/ function(top function in profiling)
It can analysed log “LwStatus” with parameters like ex-acquire-count(exclusive mode) , sh-acquire-count , block-count and spin-delay-count
Total lock acquisition request = ex-acquire-count+sh-acquire-count)
Time lock contention %= block count)/ Total lock acquisition request.
Time lock contention may provide as most of cpu time inside the function rather than spinning/ waiting for lock.
On Friday, November 12, 2021, Ashkil Dighin <ashkildighin76@xxxxxxxxx> wrote:
HiI suspect lock contention and performance issues with __int128. And I would like to check the performance by forcibly disabling int128(Maxalign16bytes) and enable like long long(maxlign 8bytes).Is it possible to disable int128 in PostgreSQL?
On Thursday, October 28, 2021, Andres Freund <andres@xxxxxxxxxxx> wrote:Hi,
On October 27, 2021 2:44:56 PM PDT, Ashkil Dighin <ashkildighin76@xxxxxxxxx> wrote:
>Hi,
>Yes, lock contention reduced with postgresqlv14.
>Lock acquire reduced 18% to 10%
>10.49 %postgres postgres [.] LWLockAcquire
>5.09% postgres postgres [.] _bt_compare
>
>Is lock contention can be reduced to 0-3%?
Probably not, or at least not easily. Because of the atomic instructions the locking also includes some other costs (e.g. cache misses, serializing store buffers,...).
There's a good bit we can do to increase the cache efficiency around buffer headers, but it won't get us quite that low I'd guess.
>On pg-stat-activity shown LwLock as “BufferCounter” and “WalInsert”
Without knowing what proportion they have to each and to non-waiting backends that unfortunately doesn't help that much..
Andres
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